1. Field of the Invention
The present invention relates to an electro-optical apparatus. Specifically, the present invention relates to an electro-optical apparatus having a color filter layer.
2. Description of Related Art
Several proposals have been made regarding methods for forming a color filter as well as materials and methods for forming a smoothing layer or the like and a color filter in electro-optical apparatus, such as, for example, liquid crystal displays. Further, methods for forming transparent electrodes on or above a color filter have been proposed, for example, by Japanese Laid-Open Patents Nos. 61-198131, 61-233720 and 61-260224 (hereinafter collectively referred to as "the first category of related art").
It has also been proposed to provide an electro-optical apparatus having a high information content and capable of displaying high-quality color images by combining the first category of related art with the construction of an electro-optical apparatus proposed by Japanese Laid-Open Patent No. 64-519 and European Patent Application No. 246,842 (the disclosures of which are incorporated herein by reference). Such a device is adapted for high information content display by combining an optical display cell and an optically anisotropic body. However, it is known that when the display content of such an electro-optical apparatus is increased, since time-division driving is used in the apparatus, electrical rounding of the waveform applied to the display portions occurs due to factors such as wiring resistance and the capacitance of the liquid-crystal layer. Such electrical rounding lowers the effective voltage applied to the liquid crystal material, thus deteriorating the level of display performance of the apparatus. In view of this problem, certain methods are proposed, for example, by Japanese Laid-Open Patents Nos. 61-273522, 63-273834 and 1-291214 (hereinafter collectively referred to as "the second category of related art").
When the first category of related art is applied to an electro-optical apparatus for displaying images by using a matrix arrangement of column electrodes and row electrodes, in order to render the apparatus capable of providing a high information content, and with a view to increasing the duty ratio during dynamic driving, transparent display electrodes should be formed on or above the color filter so as to prevent the effective voltage applied across the liquid crystal material from being lowered. However, most color filters and smoothing layers are formed of an organic resin. Accordingly, when, for example, a layer of indium oxide-tin oxide (ITO) is to be formed (i.e., to form the transparent electrodes) on a color filter by a method such as a vacuum deposition method or a sputtering method, in order to prevent the color filter or the smoothing layer from being wrinkled or damaged in other ways, it is not possible to use a substrate temperature (when forming the transparent electrode) as high as that used when electrodes are formed directly on an ordinary glass substrate. As a result, the specific resistivity of the electrodes cannot be lowered beyond approximately 1.5.times.10.sup.-4 .OMEGA..multidot.cm. Thus, the formation of an ITO layer achieves a wiring-resistance reduction only to a limited extent, and does not assure the full reduction in resistance to a desired value.
Japanese Laid-Open Patent No. 63-273834, one of the second category of related art, proposes methods for preventing effective voltage drops by forming transparent electrodes on and below a color filter layer. The pair of transparent electrodes are electrically coupled to each other by a conductor layer. However, the construction provided in Japanese Laid-Open Patent No. 63-273834 has essentially the same limitations discussed above.
The other documents in the second category, that is, Japanese Laid-Open Patents Nos. 61-273522 and 1-291214, propose disposing thin metal wires below transparent display electrodes in such a manner that the wires extend either through inter-pixel gaps of a color filter or insulating layer, so as to improve light-shielding characteristics and processing resistances. However, Japanese Laid-Open Patent No. 1-291214 has insufficient light shielding characteristics because of an opening located between adjacent thin metal films (22) which are formed around the peripheral portions of each island-shaped color filter (16). These thin metal films are connected at the edges of the island-shaped color filter (16) to a transparent display electrode (23) which is formed on the island-shaped color filter (16).
This construction has two problems, namely, low contrast at the display region and instability of orientation. These problems are exceptionally noticeable when using a twisted nematic liquid crystal material having a twist angle between 90.degree. and 360.degree.. These problems exist because of the unevenness of the color surface due to the crevices located between each island-shaped color filter. The depth of these crevices is equal to the color filter layer thickness. There is no description or suggestion in the reference to provide a smoothing layer for addressing these problems.
Japanese Laid-Open Patent No. 61-273522 has the two problems listed above as well as some additional problems. An electric erosion reaction is apt to occur, and the display reliability deteriorate, when an electrolyte such as water contacts a region of the thin metal wires that is in communication with the outside air. This occurs because the signal input reliability is obtained by using a metal lead electrode which has electrochemical activity.